Which of these numbers is prime? ${4,\ 15,\ 41,\ 51,\ 99}$
Explanation: A prime number has exactly two factors: 1 and itself. The factors of 4 are 1, 2, and 4. The factors of 15 are 1, 3, 5, and 15. The factors of 41 are 1 and 41. The factors of 51 are 1, 3, 17, and 51. The factors of 99 are 1, 3, 9, 11, 33, and 99. Thus, 41 is a prime number.